> I am trying to get USB redirection working with libvirt, but I am 
> hitting a snag. [...] I am now getting a new error:
> 
> Error setting USB device node ACL: 'Error PoliciKit error: 
> GDBus.Error:org.freedesktop.PolicyKit1.Error.Failed: Action 
> org.spice-space.lowlevelusbaccess is not registered' (0)
> 
> It fails no matter which client I use (virt-manager, virt-viewer, 
> gnome-boxes). Is there a way to fix that permission so I can redirect my 
> USB devices?

I had the same problem some time ago. It can be fixed by extending
polkit-service-type with spice-gtk, i.e.

(operating-system
  ...
  (services
    ...
    (simple-service 'spice-polkit polkit-service-type (list spice-gtk))))